1. Field of the Invention
This invention relates to data communication.
An example of a problem in data communication will be described in the context of communicating so-called Direct Stream Digital audio data. However, the present invention is applicable to other types of clocked data, such as multi-bit audio data or video data.
2. Description of the Prior Art
Direct Stream Digital (DSD) is a high-resolution single-bit audio coding system used for the so-called Super Audio CD consumer disc format. DSD was developed with a view to producing audio signals comparable to those reproduced from the best analogue formats. DSD signals can produce a frequency response from DC to 100 kHz and have a dynamic range of greater than 120 dB across the audio band.
DSD makes use of 1-bit digital audio. 1-bit oversampling converters exploit a law of information theory whereby sample width can be traded off against sampling rate to effect conversion at a given resolution. For example a 1-bit converter that oversamples at 16 times the stored sample rate can give results which are equivalent to those obtainable with a 16 bit converter with no oversampling. 1-bit oversampling converters (also known as Sigma-Delta, noise shaping or bit stream converters) measure the difference between successive audio samples rather than representing the actual value of the waveform amplitude. In DSD a significant improvement in reproduced sound quality is achieved by recording a high frequency (64Fs) 1-bit signal directly onto a super-audio CD rather than recording a 16-bit signal at frequency Fs onto a CD using pulse code modulation.
DSD systems require a high frequency audio sample clock at 64 Fs=2.8224 MHz whereas the sample clock of standard PCM systems (Fs) is 44.1 kHz. This high frequency sample clock is transmitted along with the data to facilitate accurate signal reconstruction at the receiving end. Furthermore each channel of 64 Fs DSD audio requires a transmission bandwidth of 2.8224 Mbit/s. It is a problem to provide interconnections between large-scale multi-track production equipment for DSD audio such as multi-channel ADC/DACs, DSD mixers and multi-channel DSD recorders both because of the high audio bandwidth required for the audio data interconnection and because of the difficulty of transmitting the high frequency (64 Fs) audio sample clock between devices without compromising the integrity of the signal e.g. due to electromagnetic interference from the audio data signal.
Several known audio networking systems make use of Ethernet to transmit high bandwidth audio-data between a network of audio processing devices. For example the “Magic” system proprietary to Gibson makes use of the Ethernet Media Access Control MAC layer (i.e. physical layer and data link layer) to transmit audio data at a fixed audio sampling frequency of 48 kHz using one Ethernet frame per sample period. The CobraNet audio networking system proprietary to Peak Audio also uses the Ethernet MAC layer to transmit uncompressed digital audio data between networked devices. The CobraNet system uses a 48 kHz sampling rate and allows for transmission of 20-bit and 24-bit audio data. However, none of these known systems provides an interconnection suitable for linking DSD audio devices. This is because Ethernet frame timing is completely unsuitable for transmitting a 2.8224 MHz DSD sample clock.